COVID-19 vs. Black Plague: What Do These Infectious Diseases Have in Common?

California recorded a case of the plague in August 2020, which got people wondering how it compared to COVID-19.

In August 2020, California had its first case of plague in five years. In an August 17, 2020, press release, El Dorado County officials said that the California Department of Public Health had notified them of the positive test of a resident who was recovering at home under medical care. Health officials believed the South Lake Tahoe resident may have contracted plague after being bitten by an infected flea during a dog walk.

According to the El Dorado County release, the last reported human cases of plague in California were in 2015, when two people were exposed to infected rodents or fleas in Yosemite National Park. After treatment, they both recovered. These were the first reported human cases since 2006.

"Human cases of plague are extremely rare but can be very serious," El Dorado County Public Health Officer Dr. Nancy Williams said in the release and added that plague is naturally present in many parts of California, including the higher elevation areas of El Dorado County.

Of course, in 2020, COVID-19 was the disease on everybody's radar. But with the news of the California case and other cases of plague diagnosed in China and Mongolia, many people wondered whether there are similarities between COVID-19 and the plague. Here's how the symptoms, treatments, and outcomes of both serious infectious diseases compare.

Compared to COVID-19, the Plague is Super Rare

Bubonic plague (the most common form of the plague), aka "black death," wiped out 30-50% of Europe's population in the 14th century. In the 21st century, it's much less common. In past decades, an average of seven cases of human plague, which is caused by the bacteria Yersinia pestis, have been recorded each year in the US, the Centers for Disease Control and Prevention (CDC) reports. Globally, between 1,000 and 2,000 cases are identified each year—although the true number is likely much higher.

By comparison, during the first two years of the COVID-19 pandemic, there were more than 81 million confirmed cases of COVID-19 in the US, according to the CDC COVID Data Tracker. Of those almost a million people in the US died during that timeframe.

Are COVID-19 and Plague Transmitted in the Same Way?

No. The plague is a bacterial "zoonotic infection" in domestic and wild animals, infectious disease specialist Bruce Polsky, MD, chairman of medicine at NYU Winthrop Hospital, told Health. "Humans are an incidental host, with the bacteria transmitted by flea bites—most typically rodent fleas such as those on field mice, chipmunks, squirrels, and rabbits," said Dr. Polsky. "[Plague has] been reported among rabbit hunters, for example. But it can also be transmitted through cat scratches or bites."

Charles Bailey, MD, medical director for infection prevention at Mission Hospital and St. Joseph Hospital in Southern California, told Health that plague is "transmitted from animal reservoirs among small rodents endemic in the Western US, but can also be seen in other areas of the world, especially Africa," said Dr. Bailey.

According to the CDC, a flea bite is the most common form of transmission of the bubonic plague. Other forms of the plague, septicemic plague, and pneumonic plague, are far less common in humans; more than 80% of US plague cases are the bubonic plague. People can also become infected via direct contact with an infected animal, but human-to-human transmission is rare.

On the other hand, COVID-19 is caused by the SARS-CoV-2 virus that is transmitted from person-to-person, usually via close contact (within six feet). It might also be contracted through direct physical contact, such as shaking the hand of someone who has the virus or touching a contaminated surface. Simply being near an infected person who coughs, sneezes, or talks can expose you to respiratory droplets containing viral particles, per the CDC. If those particles get into your eyes, nose, or mouth, you could become infected.

Do COVID-19 and Plague Have Similar Symptoms?

Symptoms of the plague typically appear within one week of exposure. "These include fever, headache, and the development of tender swollen lymph nodes" near the area bitten by an infected flea, said Dr. Bailey.

Fever is also a common symptom of COVID-19, and some people report headaches. Other COVID-19 symptoms include shortness of breath, dry cough, loss of taste and/or smell, conjunctivitis, skin rashes, and digestive issues like diarrhea. However, according to a December 2021 JAMA Network Open study, about 40.5% of people with confirmed COVID-19 infection are asymptomatic.

How Do the Treatment Methods Compare?

After a diagnosis is made, the plague is readily treated with antibiotics. "It's pretty easy to treat nowadays," said Dr. Polsky. The CDC recommends hospitalization as soon as someone is diagnosed with suspected plague, and antibiotic treatment should begin as soon as possible after lab tests confirm the diagnosis. People who have been in close contact with someone diagnosed with the plague can take prophylactic antibiotics to prevent the infection, added Dr. Bailey.

When it comes to COVID-19, mild symptoms can usually be treated at home by taking over-the-counter medicines to ease aches and pains and reduce a high temperature. But if symptoms are severe (for example, persistent pain or pressure in the chest, trouble breathing, or bluish lips or face), medical care may be required.

What About Potential Complications?

If the plague isn't treated quickly enough, the bacteria can spread to other parts of the body and result in more serious illnesses, such as meningitis and pneumonia. Dr. Bailey said the most serious complications arise from septicemic plague (resulting in shock and bleeding disorders) and pneumonic plague (causing shortness of breath, cough, and chest pain).

Older adults and people with underlying medical conditions are more susceptible to complications from COVID-19, such as acute respiratory failure, acute respiratory distress syndrome (ARDS), pneumonia, blood clots, and septic shock.

So Are There Any Similarities Between COVID-19 and the Plague?

Aside from overlapping symptoms like fever and headache, it's fair to say that COVID-19 and the plague are very different diseases. "The only commonality is that like COVID-19, the plague can be transmitted person-to-person by respiratory droplets," said Dr. Polsky. (According to the CDC, this type of spread hasn't been recorded in the US since 1924, although it occurs more frequently in developing countries.)

Plague killed millions of people around the world centuries ago. In the 21st century, it's safe to say that the plague is unlikely to become a global health threat like COVID-19 because it's easily prevented, and treatment exists to cure it. The plague has been around much longer, and researchers have a solid understanding of it—something they're still working on regarding COVID-19.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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